This invention relates generally to flowmeters of the vortex-shedding type and more particularly to an improved shedding body for meters of this type.
It is well known that under certain circumstances, the presence of an obstable in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are referred to as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate.
An improved form of vortex-type flowmeter is disclosed in the Burgess U.S. Pat. No. 3,589,185 wherein the signal derived from the fluid oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio insuring accurate flow-rate information over a broad range. In this meter, the obstacle assembly mounted in the flow conduit is constituted by a block positioned across the conduit with its longitudinal axis at right angles to the direction of fluid flow, a strip being similarly mounted behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This vortex street is sensed to produce a signal whose frequency is proportional to flow rate.
In Herzl U.S. Pat. No. 3,867,839, the obstacle assembly for the vortex type meter disclosed therein also makes use of a block mounted across the flow conduit, but this block has a triangular cross-section with its apex being pointed toward the incoming fluid. Obstacle bodies having other cross-sectional shapes including a circular shape are disclosed in the Bird U.S. Pat. No. 3,116,639, in the Rodley U.S. Pat. No. 3,572,117, and in the pending Herzl-Metzger application Ser. No. 670,998, filed Mar. 26, 1976.
The advantage of a vortex-shedding body having a cylindrical form is that it is physically strong, mechanically stable and offers adequate internal space within which to mount various sensing systems. But these advantages are offset by the fact that a cylinder produces an irregular shedding action and does not have a very large operating range of constant meter coefficients, to say nothing of a number of other problems which militate against the use of a cylindrical shedding body.
It is known that flat plates afford the strongest shedding phenomenon. On the other hand, flat plates are physically weak and do not provide sufficient internal space for mounting sensing systems. For example, in the Herzl U.S. Pat. No. 3,946,608, the obstacle body has a trapezoidal cross-section, and it becomes possible with this shape to mechanically transmit the vibrations of a deflectable section cantilevered from the rear of this body to an external coupling point by means of a rod passing through an internal duct in the body. As a practical matter, this is difficult to do with a flat plate.
As pointed out previously, an obstacle having a circular cross-section is the most efficient from the standpoint of strength and internal volume to accommodate a sensing system. The next most efficient shape from the same standpoint is the square or rectangle. However, the circular shape is hydraulically poor, while the square or rectangular shape, though better, is not hydraulically a very good vortex shedder.
The problems encountered with obstacle assemblies of the type heretofore known are aggravated in flowmeters of relatively small size. Thus only one company has been able to produce a vortex-type flowmeter with a flow tube diameter of less than two inches, the meter including a flat plate obstacle. But the limitations imposed on the sensing system by this obstacle shape and the concomitant hydraulic problems are such that these meters have been marginal in operation.